1. Field of the Invention
The present invention concerns a method for acquiring magnetic resonance data with different types of weightings, as well as a magnetic resonance data acquisition system with which such a method can be implemented.
2. Description of the Prior Art
Magnetic resonance technology has been increasingly used in recent years to generate cross-sectional images of the human body since, relative to other medical imaging modalities such as, for example, radioscopy with x-rays or computed tomography, it has among other things, the advantage that patient and medical personal are subjected to no ionizing radiation exposure.
Magnetic resonance (MR) technology is a known technology with which images of the inside of an examination subject can be generated. For this purpose, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths of less than 0.2 Tesla to 7 Tesla and higher) in an MR apparatus so that the subject's nuclear spins become preferentially oriented along the basic magnetic field. The examination subject is exposed to radio-frequency (RF) excitation pulses to excite nuclear magnetic resonances, the excited nuclear magnetic resonances being measured (detected) and MR images being reconstructed based thereon. For spatial encoding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a “k-space” matrix. An associated MR image can be reconstructed from the k-space matrix populated with such values by means of, for example, a multi-dimensional Fourier transformation.
A magnetization vector associated with an ensemble of spins that is forced out of its equilibrium alignment (i.e, alignment with the static magnetic field) by an RF pulse seeks to return to its equilibrium position after the RF pulse ends. The position of this magnetization vector (e.g., in degrees) relative to the direction of the static magnetic field represents the extent to which the spins have departed from their equilibrium state. The return to the equilibrium state is referred to as “relaxation”, and the return of the component of the magnetization vector parallel to the direction of the basic magnetic field to the original longitudinal direction (equilibrium) is therefore referred to as longitudinal relaxation. Since this relaxation is determined by the spin-lattice (T1) interactions, it is also referred to as spin-lattice or T1 relaxation.
There are also spin-spin interactions, designated as T2 interactions, which result in a loss of phase coherence among the magnetization vectors. This loss of phase coherence is manifest as a decay of the transverse (i.e., perpendicular to the longitudinal axis) component of the magnetization vectors, and is therefore also referred to as transverse or T2 relaxation.
By selectively manipulating different parameters that are used in magnetic resonance pulse sequences, the contributions of the T1 relaxation and the T2 relaxation to the signal from a particular tissue can be manipulated. Manipulation of these contributions has the effect of changing the contrast with which different types of tissues are represented in the resulting magnetic resonance image. In order to highlight a particular tissue type of interest, or to make features of interest more readily identifiable in the resulting magnetic resonance image, the contribution(s) of the T1 relaxation and/or the T2 relaxation is/are manipulated in an intentional manner so as to “weight” the image to set or select a desired contrast. Such images are then referred to as T1-weighted images when T1 differences dominate the image appearance and T2-weighted images when T2 differences dominate the image appearance. In T1-weighted images tissues with a long T1 are dark, whereas in T2-weighted images tissues with a long T2 are bright.
T1-weighted images and T2-weighted images are particularly suitable for use in brain imaging, in order to selectively highlight gray matter and/or white matter and/or brain pathologies. Cerebrospinal fluid (CSF) has a very high water concentration, so that it exhibits a very long T1 relaxation time as well as a very long T2 relaxation time. In T1-weighted images, therefore, CSF will produce only a small signal contribution, and thus will appear dark. In T2-weighted images, however, the same CSF will produce a very high signal, and thus will appear bright.
The magnetic resonance data acquisition parameters also can be selectively adjusted so that the resulting image is neither T1-weighted nor T2-weighted, and thus is primarily influenced solely by the number of signal-producing nuclei per volume element. If hydrogen atoms are being imaged, this type of weighting is referred to as proton density (PD) weighting. For imaging other atoms, this type of weighting is referred to as spin density weighting.
A magnetic resonance data acquisition pulse sequence that begins with an approximately 90° excitation RF pulse followed by an approximately 180° refocusing RF pulse is commonly referred to as a spin echo pulse sequence. Many types of spin echo pulse sequences are known, and spin echo pulse sequences represent a basic family or category of magnetic resonance pulse sequences.
Highly sophisticated spin-echo pulse sequences include a single-slab 3D turbo or fast spin-echo pulse sequence known as, among other names, SPACE (Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions). Pulse sequences of this type allow an extremely large number of refocusing RF pulses (e.g., >300) by using a refocusing RF pulse train exhibiting pulses with respectively different flip angles (<)180° throughout the duration of the echo train. An example variation of the flip angle following a given excitation RF pulse in such a pulse sequence is illustrated by the curve shown in FIG. 2. The shape of such a curve is intentionally designed to achieve desired signal strengths for different types of tissue, and is referred to as a flip angle evolution. Such an evolution is designed for obtaining a specific contrast (such as in proton density-weighted images or T1-weighted images or T2-weighted images) between the imaged tissues. FIG. 3 shows an example of normalized signals strengths respectively for CSF, gray matter and white matter, with respect to time, obtained in a T2-weighted image using the flip angle evolution shown in FIG. 2. With this echo train, an optimal T2-weighted contrast between the tissues is obtained by setting the echo time around the middle portion of the echo train.
The example of FIGS. 2 and 3 shows a signal evolution that has an echo-train duration on the lower end of those used for SPACE-type pulse sequences. Echo-train durations up to 1 second are commonly used and durations of several seconds may be used for certain applications. For obtaining a proton density-weighted contrast, however, the echo train is typically no more than approximately several hundred milliseconds. The reordering of the encoding is such that earlier echoes are entered in the central portion of k-space. Therefore, the proton density-weighted evolution is essentially the first part of the evolution that occurs for T2-weighted contrast.